When the Montana Department of Transportation (MDT) began reconstruction of a highway drainage channel near Roundup, 35 mi. north of Billings, it needed to design the project to withstand the state’s harsh variations in climate. In Montana’s Big Sky country, extreme topography causes seasonal temperature fluctuations ranging from -40ºF in the winter to desert highs of 105ºF. Precipitation ranges from blizzards and violent thunderstorms to extended drought.
TRM installationThe channel, near US Highway 87, is one of the primary means of drainage for the Bull Mountains watershed, conveying large flows from spring snowmelt and stormwater runoff. A number of events converged to damage the channel. In the fall of 1993, a fire in the Bull Mountains destroyed most of the area’s vegetative cover. The following season, before any new vegetation was established, the area was inundated by a 100-year storm event. To further hinder the reestablishment of vegetation in the channel, 100-year storms occurred in the following two years as well, a rare sequence of events for this semiarid region. The storms and the resulting concentration of runoff from the mountains and highway, in addition to runoff from an 84-in. culvert that diverted runoff into the channel, caused progressive undercutting of Highway 87 until the structural integrity of the road itself became a concern. Bill Wandersee, project design supervisor for MDT, and Clyde Bennett, an erosion control specialist with the Roscoe Steel Culvert Company, sought a channel liner system that would provide immediate protection and also be capable of withstanding flows from future storms of the same magnitude. MDT had specified revegetating the channel. “With it being at the bottom of a road, the last thing they wanted was to put in riprap and have cars go off and down into it,” says Bennett. “They wanted a soft-armor approach.”
Channel after one growing season
Channel after two growing seasonsIn this region of Montana, at least two growing seasons are required before a mature vegetative stand becomes established in the fine, sandy soil. Because of poor soils and limited precipitation, the mature plants are often widely spaced. The silt and clay portions of the soil are susceptible to erosion, and because this fraction of the soil also helps regulate the nutrient level of the soil, loss of these particles can result in a deficiency of nutrients and slower vegetative growth. “We have very little topsoil here, and we can’t afford to have much of it leave, let alone take seed with it,” notes Bennett.He recommended a composite turf reinforcement mat, C350 manufactured by North American Green, to provide immediate protection and support revegetation. A matrix of coconut fiber provides about three years of erosion protection and mulching action to aid seed germination and plant growth. “The area heading into the Bull Mountains is sandstone, and about the only soil you get is what crumbles from the sandstone or may wash down through the drainage area and deposit within the matting itself,” Bennett points out. “You’re somewhat dependent on seeds and weeds that blow in to start growing in from the top, as well as what has been seeded underneath.”Even in the first growing season after the mat was installed and the channel reseeded, vegetation flourished. The mat’s three layers of UV-stabilized polypropylene netting provide long-term stability; five years after installation, vegetation has become thoroughly entangled in the net-even though, as Bennett notes, there have been several years of drought. “I grew up in the Midwest where, if you wait a few days, something will grow. That’s not the case in the West. In semiarid areas, it’s so important to use a natural product, such as straw or coconut, combined with that turf reinforcement portion of the matting,” he states, adding that the organic portion of the mat helps plants take advantage of whatever moisture is available.The net also provides supplemental erosion protection for the exposed soil between the mature plants, as the native vegetation growing in the channel is of lower density than in many other climates. “Once you hit that third year,” says Bennett, “if the vegetation’s there, it’ll stay. It will come back.”
